last time i made type L double entry visa, and i already visit china on 12 April (first entry).

then my last entry before its expired is 12 September. So i'm going back there again at 11 September for study. What i want to ask is, am i allowed to stay in china after 12 september? (After that i will extend my L type, or make student type) because i saw that the duration of each stay 30 days after entry. Thanks